The developers of a type of malicious software that encrypts a computer's files and demands a ransom have fixed an error security experts said allowed files to be recovered without paying.

The malware, called TorrentLocker, popped up last month, targeting users in Australia, according to iSight Partners, a security consultancy. It now appears to be also geo-targeting victims in the U.K.

TorrentLocker's developers ironically made a similar mistake as the creators of another ransomware program, CryptoDefense. Researchers found earlier this year that CryptoDefense left a decryption key on a person's computer, although the error was soon fixed.

Earlier this month, researchers with the consultancy Nixu found that TorrentLocker used the same keystream to encrypt all of a computer's files. That was a mistake, as a keystream should never be used more than once, according to a writeup on the SANS Institute blog.

"As the encryption was done by combining the keystream with the plaintext file using the XOR operation, we were able to recover the keystream used to encrypt those files by simply applying XOR between the encrypted file and the plaintext file," they wrote.

With the error out in the open, it was only a matter of time before it was fixed.

Richard Hummel, a senior technical analyst with iSight, wrote that a variant of TorrentLocker without that bug has now been found, which shows the "extremely high pace of innovation of our collective adversaries."

The latest version also scans profiles in the Thunderbird email client for email addresses and passwords, he wrote. "This will almost certainly be used to further the spam campaign for TorrentLocker," he wrote.

TorrentLocker asks for US$500 to unlock the files, payable in bitcoin. Hummel wrote that although the percentage of people who pay is low, a look at the bitcoin address associated with TorrentLocker showed that the attackers are making "many bitcoins," he wrote.